This invention relates to a light emitting device comprising a light emitting diode or laser diode (LED) and an excitable phosphor, which is spaced from the LED chip by a light transmissive medium for a more uniform emission of the light output. The invention finds particular application in combination with a UV/Blue LED and a phosphor or blend of phosphors, for converting LED-generated ultraviolet (UV) and/or blue light into white light for general illumination purposes. It should be appreciated, however, that the invention is also applicable to the conversion of light from other light sources to light of a different wavelength.
The advent of GaN-based epitaxial structures has lead to the development of UV and/or blue (xe2x80x9cUV/bluexe2x80x9d) light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LEDs). By combination of the light emitting device with one or more phosphors, generation of visible light (e.g., red, blue, or white light) is achieved. The phosphor transforms a portion of the UV and/or blue light into light of longer wavelength, for example, by employing a UV/blue absorbing, yellow emitting phosphor, such as Y3Al5O12xe2x80x94Ce3+ (commonly referred to as YAG-Ce), obtainable from Nichia Chemical Company.
To form white light, the YAG-Ce phosphor converts a portion of the LED blue light into yellow light. This produces a white field with a color rendering index (CRI) of about 77 and a color temperature ranging from about 6000 K to 8000 K. For some applications, conversion of UV/blue light from an LED to visible light using phosphors may be more attractive than the direct application of visible LEDs. Such UV/blue LED phosphor devices, for example, other the opportunity to encompass a wider color range, which is important for display as well as for illumination applications. Other phosphors convert the light to different wavelengths. Thus, the color of the light can be modified by combining two or more phosphors.
LEDs, including the blue and UV emitting types, comprise a generally rectangular chip or die, formed from a semiconductor material, that radiates in a non-uniform fashion. In particular, the intensity of light generated at some regions of the front surface of the attached die (e.g., at the bond pads) is only a small fraction of the light emitted from the translucent sides and the remainder of the front (and from the back, in the case of a flip chip). When the die is coated with a layer of phosphor, the non-uniformity in the radiation emitted by the LED results in non-uniform excitation of the phosphor and causes non-uniformity in the color and/or intensity of the light emitted by the device.
In regions where the intensity of the LED emission is relatively high, the phosphor material may become overheated. Many phosphors are temperature sensitive, in that they suffer a temporary reduction in their emission efficiency when they become hot. The effect is reversed when the phosphor cools. The reduction in phosphor efficiency reduces the overall efficiency of the light source and results in a patchy emission- i.e., one which is not uniform in its angular distribution, and/or which varies in color.
Additionally, in areas of particularly high LED emission, the phosphor material can become saturated, i.e., it ceases to show a linear increase in emission as the LED intensity increases. At a certain level of intensity, all the phosphor molecules are in an excited state and thus further increases in LED intensity cannot be utilized by the phosphor.
Another problem with LED emission is that the die attach materials tend to block light emitted from the sides of tile LED. These materials are used to attach the LED to a cup or other support and to provide thermal transfer away from the die to a heat sink. The typical die attach materials used are silver epoxy or ceramic filled epoxy, which are thermally cured. During the die attach process, there is an inherent wicking of the attach material up the sides of the die. It is difficult to apply a sufficient amount of the die attach material to bond the device in place and provide thermal conduction without some wicking taking place. Particularly in the case of LEDs with sapphire substrates, this causes significant degradation in the light output, since a large proportion of the light output from Such LEDs is through the sides.
The present invention provides a new and improved light source, and method of formation, which overcomes the above-referenced problems and others.
In an exemplary embodiment of the present invention, a light source is provided. The light source includes a light emitting component which emits light and a layer of a phosphor material positioned to receive light emitted by the light emitting component. The phosphor material converts at least a portion of the light to light of a different wavelength. A layer of a light transmissive material spaces the phosphor material from the light emitting component.
In another exemplary embodiment of the present invention, a light source is provided. The light source includes a light emitting component which emits light of at least a first wavelength. A phosphor-containing material is positioned to receive light emitted by the light emitting component and convert the light of the first wavelength to light of a second wavelength. A layer of a light transmissive material spaces the phosphor material from the light emitting component. The layer of light transmissive material has a thickness which is greater in regions where the intensity of the light emitted by the light emitting component is higher than in regions where the intensity of the light emitted by the light emitting component is lower.
In another exemplary embodiment of the present invention, a method of improving a light source emission is provided. The method includes providing a layer of a curable material on a light emitting component and curing the curable material to form a layer of a light transmissive material. The method further includes forming a layer of a phosphor material on the light transmissive material, the light transmissive material spacing the phosphor material from the light emitting component.
One advantage of the present invention is that light is produced with a relatively uniform color over a wide range of viewing angles.
Another advantage of the present invention is that the intensity of the converted light output is increased due to the optimal spacing of the phosphor from the die.
Another advantage of the present invention is that the phosphor is thermally insulated from high temperatures generated near the LED junction.
Another advantage of the present invention derives from the ability to form the phosphor in an even, thin layer with a greater surface area than that of the die.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.